The present invention relates to a method and apparatus for controlling defrosting of an evaporator in a heat transfer system, particularly but not exclusively in a refrigeration system in which there is a forced airflow over the evaporator.
FIG. 1 shows in cross-section a refrigerated display cabinet 2, which is one example of such a refrigeration system. The cabinet 2 has a number of shelves for displaying chilled food or drinks. The cabinet 2 is open at the front (to the left in FIG. 1) to allow shoppers easy access to the contents of the shelves 4. The contents are cooled by air blown by a fan 6 over an evaporator 8 of the refrigeration system, which cools the air. As shown by the arrows in FIG. 1, the air leaves the evaporator 8, is forced up a duct 10 and escapes through small vents 12 so that some of the air flows over the contents of the shelves 4.
Most of the air passes through an end aperture 14 at the top of the cabinet 2 and falls as a curtain of cold air down the open front of the cabinet 2 and into an inlet 16, to be recirculated over the evaporator 8. The air curtain hinders the warm ambient air from entering the cabinet.
However, some of the ambient air is drawn into the inlet 16. The ambient air includes water vapour which condenses and freezes on the evaporator 8 to form frost. The frost impedes the passage of air over the evaporator 8 and reduces the efficiency of heat exchange between the evaporator 8 and the air. If the frost is allowed to build up, the rate of airflow will be reduced sufficiently to prevent the air curtain from forming and the internal temperature of the cabinet will rise. Furthermore, the efficiency of the refrigeration system will be reduced, leading to higher running costs.
For these reasons, it is necessary to defrost the evaporator 8 in such refrigeration systems every few hours. There are different conventional methods by which this can be done. In the xe2x80x9cair overxe2x80x9d or xe2x80x9coff cyclexe2x80x9d method, the refrigeration is stopped and the evaporator 8 is defrosted by air at ambient temperature passing over it. In the electric defrost method, electric heating elements are provided around the evaporator 8. During a defrost cycle, the flow of refrigerant through the evaporator 8 is stopped and the electric heating elements are switched on, thereby melting the frost; the fan 6 may be switched off.
In the gas defrost method, gas is passed through the evaporator so as to warm it and melt the frost. The gas may be directed from the outlet of the compressor of the refrigeration system through the evaporator, so that the evaporator 8 acts temporarily as a condenser and the refrigeration cycle acts in reverse to release heat from the evaporator 8. This is known as the xe2x80x9chot gasxe2x80x9d method.
Alternatively, the gas may be taken from the top of the receiver of the refrigeration system, in which the refrigerant is stored before passing through the expansion valve. This is known as the xe2x80x9ccool gasxe2x80x9d method, since the refrigerant has passed through the condenser and is cool.
During a defrost, the air temperature inside the cabinet 2 rises above the normal storage temperature, and the contents are subject to xe2x80x9ctemperature shockxe2x80x9d. The effect of this temperature shock is to reduce the shelf life of perishable goods. Moreover, the defrost cycle consumes a significant amount of energy, typically around 10% of the total energy used in refrigeration.
Therefore defrost cycles should not occur too frequently, but neither should they occur so infrequently that the refrigeration efficiency of the cabinet 2 is impaired.
In one conventional method of defrost control, a defrost is initiated periodically at intervals sufficiently short to prevent the evaporator 8 from frosting up completely and thereby blocking the flow of air, even at the maximum absolute humidity for which the cabinet 2 is designed. This interval is typically between 6 and 8 hours. However, when the absolute humidity is less than its maximum, defrosts occur more frequently than required.
It is therefore desirable to initiate a defrost xe2x80x9con demandxe2x80x9d, that is to say only when it is needed.
The document U.S. Pat. No. 5,046,324 discloses a defrost control method in which defrosting is initiated periodically, but a defrost operation is omitted when the total proportion of time spent operating the refrigeration cycle during the last refrigeration period is less than a predetermined value.
The present applicant""s earlier patent publications U.S. Pat. Nos. 5,813,242, GB-A-2314915 and EP-A-816783 disclose a defrost control method and apparatus in which a defrost is initiated in response to the detected superheat at the outlet of an evaporator. In a disclosed example, a controller controls the flow of refrigerant through the evaporator so as to keep the temperature of the thermal load constant. However, if the detected superheat at the outlet of the evaporator is too low, the controller enters an override state so that the flow of refrigerant is reduced, thereby raising the superheat. If the period spent in the override condition exceeds a predetermined level, a defrost is initiated.
The present applicant""s patent publication no. GB 2348947 discloses a defrost control method and apparatus in which the flow rate through an evaporator is regulated to maintain a desired level of superheat at the outlet. An initial flow rate is measured immediately after a defrost. As the evaporator frosts up, the flow rate falls as the rate of heat transfer into the evaporator falls. When the flow rate has fallen to a predetermined fraction of the initial flow rate, defrosting of the evaporator is triggered.
According to one aspect of the present invention, there is provided a method for controlling defrosting of an evaporator in a heat transfer system, including controlling the flow rate of refrigerant through the evaporator so as to maintain the superheat of refrigerant at or about an outlet of the evaporator substantially constant, and initiating defrosting of the evaporator in response to the fluctuation of the flow rate through the evaporator satisfying a predetermined criterion which indicates that the flow has become unstable.
The flow rate may be controlled automatically by a thermostatic expansion valve. Alternatively, the level of superheat is detected by a sensor and an electronically controlled expansion valve is controlled to keep the superheat at a predetermined level.
The flow rate may be sensed by a flow rate sensor. Alternatively, the flow rate is derived from the degree or period of opening of the expansion valve. Alternatively, the fluctuation in the superheat at the outlet of the evaporator may be measured. An approximate measure of the superheat may be used, derived from the difference between the temperature at the outlet and at a point upstream of the outlet within the evaporator.
Preferably, the flow of refrigerant through the evaporator is switched on and off in response to the sensed temperature of a thermal load rising above a predetermined maximum temperature and falling below a predetermined minimum temperature respectively. The fluctuation of the flow rates is detected only during the period in which the flow is switched on.
The present invention also encompasses apparatus and/or software arranged to carry out the above method.
The fluctuation of the flow rates has been found to give more reliable indication of the degree of frosting of an evaporator than the prior art methods. Moreover, the algorithm based on fluctuation of flow rates is relatively simple to set up and operate, and can be added to an otherwise conventional control apparatus. Measurement of the fluctuation in superheat is particularly advantageous as there is no need to install a flow meter; instead, sensors already required for flow regulation may be used, or only additional temperature sensors need be installed.